Police arrested a Zambian opposition leader on Thursday and said he would be charged with defaming President Edgar Lungu, an offense that carries a maximum five-year prison term.

Officers did not say how Saviour Chishimba, the head of the United Progressive People (UPP) party, had defamed the president.

Chishimba made headlines in July, criticizing Lungu's decision to impose emergency powers and suggesting the president should go to hospital for mental tests.

Political tensions in Zambia, once seen as one of Africa's more stable and functional democracies, have been rising since main opposition leader Hakainde Hichilema was arrested on treason charges in April.

Lungu said on July 6 that he had invoked emergency powers to deal with "acts of sabotage" by his political opponents, after fire gutted the southern African country's biggest market.

The president said the measure would safeguard investments in Africa's second-biggest copper producer, which is in talks with the International Monetary Fund over a financial aid package.

Police said Chishimba's offense took place between July 1-8.

"He has since been detained in police custody and will be charged once the officers are done with processes," said police spokeswoman Esther Mwaata-Katongo.